First of all what are you looking to accomplish? RAID is designed to protect against a hardware failure. That is to say, enable the system to remain operational without the loss of data in the event of a drive failure. Depending on your situation and the amount of data will determine the RAID type that would be best 1,5,6, etc).

If this is what you are looking to protect against then there are two options, adding a RAID controller (if your system doesn't have an onboard) and appropriate drives., or using software RAID. In either case you are going to have to migrate your data to the RAID array. Most RAID setups require drive configuration ebfore sue so you may be better off elaving your OS on it's current drive and setting up the RAID for the data drives only.

If you are looking to safeguard data, then backing up to a separate drive is a better solution. ideally you have at least two backups, one offsite. If the data is critical to operations, and needs high availablility, then a combination of RAID and backups is the preferred solution.